The invention relates to a combination of a polyol ester oil additive and an imide oil additive which markedly improve the sludge dispersancy-varnish inhibiting properties of lubricating oils employed for crankcase lubrication of internal combustion engines.
There are two principle environments which are encountered by automotive crankcase lubricants, i.e. cyclical high and low temperatures from stop-and-go driving and continuous high temperatures from extended operation of the automobile over long distances. Each of these environments poses a primary problem which should be solved if a lubricant is to be regarded as satisfactory. These problems result from varying proportions of foreign particles including dirt and oil decomposition products in the lubricant from breakdown of the oil. The presence of water and precursors of sludge in lubricating oil seems to depend largely on the operating temperature of the oil. Although at high temperatures the water will be evaporated, breakdown of the oil is accelerated. At low temperatures, water will accumulate and so consequently will provoke the sludging. In ordinary stop-and-go driving, the crankcase lubricant will be alternately hot and cold so that the formation of sludge is a serious problem.
Another principle problem in addition to the sludge which must be overcome by a satisfactory lubricant is varnish deposition which results from the operation of the engine at continuous high temperatures. In such an environment, oil breakdown results in the formation of acidic materials which in themselves corrode the metal surfaces of the bearings, pistons, etc., as well as catalyze the decomposition of the lubricating oil which decomposition is manifested in hard, carbonaceous deposits which accumulate in the piston ring groove and form a varnish on the piston skirts and other metal surfaces.
During the past decade, ashless sludge dispersants have become increasingly important, primarily in improving the performance of lubricants in keeping the engine clean of deposits and permitting extended crankcase oil drain periods while avoiding the undesirable environmental impact of the earlier used metal-containing additives. Most commercial ashless dispersants fall into several general categories. In one category, an amine or polyamine is attached to a long-chain hydrocarbon polymer (the oil solubilizing portion of the molecule), usually polyisobutylene through an acid group, such as a dicarboxylic acid material such as polyisobutenyl succinic anhydride, by forming amide or imide linkages such as described in U.S. Pat. No. 3,272,746 and may include the reaction product of such materials with boron (see U.S. Pat. No. 3,254,025).
Another category is the oil-soluble polyol esters, usually the reaction products of hydrocarbon substituted succinic anhydride, e.g., polyisobutenylsuccinic anhydride, with polyols e.g. pentaerythritol, which have been taught as ashless sludge dispersants (see U.S. Pat. No. 3,381,022). Further, the reaction products of said polyol esters with boron compounds are said to be useful as detergents for lubricating oils (see U.S. Pat. No. 3,533,945).